At present, projectors for business purposes have been widely spreading. Projective TV sets, which use a liquid-crystal display or the like, have been under development to replace conventional displays which project images displayed in a Braun tube onto a screen. A projective TV set, including a screen, needs a reflector of relatively large size, because (i) a projective TV set provides a relatively large space for devices, and (ii) a light source of long arc length (large light-emitting section) and reflector of long focal distance are frequently used to extend lamp life. On the other hand, it is particularly important to decrease size and weight of a projector for transportation. JP-A-2002-244199, for example, discloses a method for positioning a concave plane, which corrects light passing near the light axis of the light source to travel almost in parallel to the light axis, between an elliptic reflector and second focus.
How a reflector works is described by referring to FIGS. 7 and 8. FIG. 7 illustrates one example of reflector widely used for projective TV sets, where (a) is an oblique view, and (b) a cross-sectional view showing a plane which includes the light axis. FIG. 8 illustrates one example of small-size reflector widely used for projectors, where (a) is an oblique view, and (b) a cross-sectional view showing a plane which includes the light axis. The light beams are shown in each of the cross-sectional views. These figures illustrate the reflectors on the same scale to allow direct comparison between their sizes.
Referring to FIG. 7, the reflector comprises the light source 1, reflector 2 with a parabolic cross-section, and light axis 3. The light source is put into the parabolic reflector through a hole provided at the apex and positioned at the focus of the parabolic plane. The reflector 2 has a reflection plane rotationally symmetrical around the light axis 3. As a result, the light beams emitted from the light source 1 positioned at the focus of the parabolic plane of the reflector 2 are reflected by the parabolic plane to travel in parallel to the light axis. On the other hand, the small-size reflector 2 widely used for projectors, illustrated in the oblique view (a) in FIG. 2, the reflective plane is not rotationally symmetry around the light axis 3 passing through the light source 1, because the reflector peripheries are cut to decrease projector size. In addition to these physical arrangements, the reflector 2 is also devised to have a shortened focal distance to decrease its size.
Various reflector glass compositions have been proposed. JP-B-7-92527 discloses crystallized glass with a solid solution of β-spondumene or β-eucryptite as the major component, JP-A-2002-109923 crystallized glass with celsian as the major crystal phase, and JP-A-2001-249206 crystallized glass with β-quartz having a crystallite diameter of 400 nm or less as the major crystal.